A speaker converts an electrical energy into a mechanical energy to generate sound, using a voice coil present in an air gap according to Fleming's left hand rule. Recently, with the wide spread of a small electronic device requiring a small internal speaker, such as a smartphone, there are increasing demands for a small and slim micro-speaker.
As the micro-speaker is limited in terms of a size, shape, location of a sound emitting hole, etc., a structure for obtaining high sound quality in a limited space has been taken into account. Especially, an enclosure micro-speaker module has advantages in that the micro-speaker is provided in an enclosure casing serving as a resonance space, this enclosure casing is mounted in an electronic device, and the sound generated by the micro-speaker is resonated in and emitted from the enclosure casing, which can thereby reduce sound interferences and improve sound quality and sound volume. In particular, the resonance space of the speaker is a critical factor in the low frequency characteristics, and the larger the resonance space is, the more easily the low frequency sound can be reproduced and the more the reproducible frequency range can be increased.
Recently, a micro-speaker having an air adsorbent starts to be developed to further enhance such low frequency characteristics. Zeolite or activated carbon is put into an enclosure to define a virtual back volume, i.e., a resonance space, using adsorption and desorption of air molecules. European patent 2424270, U.S. patent publication 2015-0358721, and U.S. Pat. No. 8,687,836 disclose a speaker which uses zeolite to enhance the low frequency sound characteristics.
However, the conventional micro-speakers having zeolite have adopted expensive materials with a high mass ratio of silicon to aluminum so as to achieve goals such as improvement in the low frequency sound quality. In general, the higher the mass ratio of silicon to aluminum is, the more hydrophobic zeolite is, so only the mass ratio of silicon to aluminum has been considered as a major performance index for zeolite. It is because the hydrophobic air adsorbent less adsorbs water molecules and thus more adsorbs air molecules. Nevertheless, increasing the mass ratio of silicon to aluminum significantly increases a process cost and thus a unit cost, as a result of which the micro-speaker having the air adsorbent has not been widely spread in spite of its superior performance.